Are you referring to this set: http://rebrickable.com/sets/7470-1/space-shuttle-discovery-discovery-2003 I'm not aware of any other official Mars Rovers.

I think the smaller and cheaper versions are more likely to get through the Lego Cuusoo criteria. Fewer people would buy a more complicated and expensive Mindstorms one.

Haha, thanks for crashing my website :) As the creator of Rebrickable, it's awesome to be here... I'm stuck at work now with little means to manage the site, but please try again later if its down!